This invention relates to the preparation of copolycarbonates, and more particularly to their preparation by a method requiring enhancement of crystallinity.
Solid state polymerization (hereinafter sometimes "SSP") of polycarbonates is disclosed, for example, in U.S. Pat. Nos. 4,948,871, 5,204,377 and 5,214,073, the disclosures of which are incorporated herein. It involves a first step of forming a precursor polycarbonate, typically a prepolymer formed by melt polymerization (i.e., transesterification) of a dihydroxyaromatic compound such as bisphenol A with a diaryl carbonate such as diphenyl carbonate; a second step of enhancing the crystallinity of the prepolymer; and a third step of building the molecular weight of the crystallized prepolymer by heating to a temperature between its glass transition temperature and its melting temperature. Use of this polymerization method is of increasing interest by reason of its effectiveness and environmental benefits.
The second or crystallinity enhancement step of this method is performed, according to said patents, by solvent treatment or heat treatment. As described, the solvent treatment method may in fact employ a good or poor solvent for the prepolymer, with contact involving either the liquid or vapor form thereof. Illustrative "solvents" include aliphatic aromatic hydrocarbons, ethers, esters, ketones and halogenated aliphatic and aromatic hydrocarbons. It is sometimes considered important that the "solvent" be allowed to permeate into the prepolymer. The methods of crystallinity enhancement which involve permeation apparently require that the crystallinity of the entire prepolymer mass be increased to a value of at least 5% as determined from powder X-ray diffraction patterns.
One major use area for polycarbonates in recent years has been in the fabrication of optical data recording media, including optical disks as exemplified by compact audio disks and CD-ROM disks used in computers. The data on an optical disk are read by a plane polarized laser beam, associated with a similar reference beam polarized in a perpendicular direction. For accurate reading, it is necessary to minimize differences in phase retardation of these laser beams upon passage through the disk.
In the further development of optical disks, particularly read-write disks and disks capable of storing larger amounts of data, various physical factors become important. One such factor which is closely related to the storage capability of the disk, is its birefringence, i.e., the difference between indices of refraction of light polarized in two directions perpendicular to each other. Birefringence has several components, some of which are related to the molecular structure of the polycarbonate from which the disk is fabricated.
Copolycarbonates containing units derived from phenylindanols, particularly 6,6'-hydroxy-3,3,3',3'-tetra-methyl-1,1'-spiro(bis)indane (hereinafter designated "SBI") and 1,1,3-trimethyl-3-(4-hydroxyphenyl)-5-hydroxyindane (hereinafter designated "CD-1") and their isomers, are characterized by significantly lower birefringences than homopolycarbonates of the commonly employed monomer 2,2-bis(4-hydroxyphenyl)propane, or "bisphenol A". However, it has been very difficult to prepare such copolycarbonates by SSP since the presence of SBI or CD-1 units apparently inhibits crystallinity enhancement.
A method of crystallinity enhancement which is successful with copolycarbonates of such monomers as SBI has been disclosed. It includes a step of contacting a precursor polycarbonate, most often a bisphenol A homopolycarbonate, with a monomeric or polymeric source of other structural units such as SBI, under conditions promoting incorporation of said structural units by reaction to form a precursor copolycarbonate, combined with simultaneous or subsequent conversion of said precursor copolycarbonate or either reactant to a polycarbonate of enhanced crystallinity. This method, however, is uniformly successful only when the material to be crystallized is in powder form. Since polymers are often conveniently handled as pellets rather than powder, it is of interest to provide a crystallinity enhancement method which is effective with pellets.